1. Field of the Invention
The present invention relates to a wet-type developing apparatus for use in an electrostatic recording type or electrophotographic type image forming apparatus such as a printer, a facsimile machine, a copying machine or the like, and an image forming apparatus using the same.
2. Description of the Related Art
In a wet-type developing process of developing an electrostatic latent image on a latent image carrying member with liquid toner containing carrier liquid and toner particles dispersed in the carrier liquid, there has been hitherto adopted such a method that in order to efficiently exclude surplus solvent (carrier liquid) on a developed image area and a liquid toner layer on a non-image area, a squeeze roller is moved with being brought into contact with the latent image carrying member or keeping a predetermined gap interval (distance) between the squeeze roller and the latent image carrying member.
For example, in the case of an apparatus described in Japanese Laid-open Patent Application No. Sho-48-79644, a squeeze roller and a photosensitive drum are moved in the same direction while the squeeze roller is pressed to the photosensitive drum, and the same polarity potential as toner is applied thereto to: prevent offset. Further, in the case of an apparatus described in a Japanese Registered Patent No. 2915031, a liquid toner layer having a thickness of several hundreds xcexcm is made to invade into the gap of about 50 xcexcm between a squeeze roller and a photoreceptor medium, variation of the viscosity of liquid toner being used is detected, and the revolution number of the squeeze roller is controlled in accordance with the revolution number. In this case, no bias voltage is applied to the squeeze roller.
Further, in the case of an apparatus described in Japanese Laid-open Patent Application No. Hei-8-179633, carrier liquid is squeezed in a squeeze gap which is narrower than the developing gap, and a gap which is larger than the squeeze gap is provided between a photosensitive drum and an electric field forming roller in the next step. When the toner layer passed through the above gap, the toner particles are cohered by discharge current occurring in the gap between the electric field forming roller and. the toner layer. In this case, the electric field forming roller is confronted to the toner layer so that the carrier liquid is surely prevented from getting contact with the electric forming roller. Therefore, no shearing stress is applied to a toner image, and thus an image having no toner dispersion can be achieved.
In this case, it is particularly necessary to keep a sufficient gap so that the toner image is prevented from coming into contact with the electric field forming roller due to attraction of the toner image to the electric field forming roller. Further, at the squeeze roller side of the front stage, the gap is narrower than the thickness of the developed toner layer, and shearing stress to exclude the surplus carrier liquid is applied. That is, the exclusion of the liquid and the cohesion of the toner particles are performed by two rollers.
As another case, the specification of U.S. Pat. No. 3,957,016 discloses that the intermediate potential between an image portion potential and a non-image portion potential is applied to a regulating roller to clean the base portion, and the image portion is slightly compressed.
Further, according to Japanese Published Patent No. Hei-4-503265, in order to remove liquid on a-non-image portion after the development and reduce the thickness of the liquid, a squeeze roller is provided and the value of bias to be applied to the squeeze roller is set to the intermediate potential between the non-image portion potential and the image portion potential. Further, a toner hardening roller to which a voltage is applied is confronted at the downstream side of the squeeze roller while the liquid toner is interposed on the latent image carrying surface between the transfer position and the toner hardening roller. The toner hardening roller is designed so that the liquid is not substantially removed from the tone image, and the toner image is afterwards transferred onto a transfer medium by electric field.
According to a method described in Japanese Laid-open Patent Application No. Hei-7-301997, a squeeze roller is moved in the opposite direction to a latent image carrying member, and the liquid adhesion amount on the surface of the squeeze roller before the squeeze roller comes into contact with the liquid toner layer after development is set to 0.1 xcexcm (micrometer) or less, whereby the thickness of the toner layer after squeezed is reduced and a toner layer suitable for the transfer step is achieved. No provision is given to the velocity ratio of the squeeze roller and the electric field at the squeeze portion. In any case, the parts are fabricated under the condition that the liquid is interposed between the squeeze gap. That is, the construction is designed so that liquid is interposed between the gap even when no external electric field exists at the squeeze portion.
Japanese Laid-open Patent Application No. Hei-10-73997 discloses the following system. Representing the image portion potential by V1, representing the non-image portion potential by V2, representing the developing bias by Vdr and representing the squeeze roller bias by Vrr, the following inequality is set: Vrr greater than V2xe2x89xa7Vdr for V1 greater than V2, and also the following inequality is set: Vdrxe2x89xa7V2 greater than Vrr for V1 less than V2. According to this system, toner is attached to the non-image portion by the development, but it is withdrawn from the non-image portion by the squeeze roller.
In the above-described prior arts, the liquid toner image is generally transferred onto a transfer medium such as paper or the like by electrophoresis. The above-described prior arts describe only the case where the thickness of the toner layer which is formed on the latent image carrying member and contains the liquid carrier after the development is larger than the gap between the squeeze roller and the latent image carrying member, or pay no attention to the thickness of the toner layer.
That is, the above-described prior arts adopts such a design that a large amount of surplus liquid toner layer adheres to both of the image portion area and the non-image portion area on the latent image carrying member and thus most of the surplus liquid toner layer is first excluded at the nip entrance port of the squeeze roller and latent image carrying member. This design is equivalent to such a design that the thickness of liquid is reduced. Accordingly, in the conventional methods, even toner particles which should originally adhere to the image portion flow out from the surface of the latent image carrying member, and thus a finally formed image is usually short of the image density. Further, since excessive shearing stress is applied to the liquid toner layer in even the gap, toner dispersion sometimes occur at the leading end or trailing end of the toner image. Still further, even when consideration is given not to apply strong shearing stress, the compression of the carrier liquid in the liquid toner layer adhering to the image portion is insufficient, and thus the solid content (component) rate of the toner layer cannot be increased. This causes the toner image to be squeezed and flow out in the next transfer step. Further, this causes such a disadvantage that the drying time in the fixing step is long. Further, there is such a disadvantage that the transfer based on the pressure using;no electrostatic electric field and the adhesive force cannot be executed as a transfer method.
As a method of forming a color image may be considered a method of squeezing a first color toner image, to achieve a sufficient solid content rate on the latent image carrying member, subsequently electrifying the overall surface on the latent image carrying member on which the first color toner layer is carried out, and executing a series of steps of performing light exposure, development and squeezing for a second color toner image to superpose two color toner images on the latent image carrying member. If the same steps until the squeezing step as described above are executed for plural other color toner images on the latent image carrying member, a full color image can be formed on the latent image carrying member. Accordingly, there can be achieved a process of collectively transferring two or more color toner images on an intermediate transfer medium or a recording medium such as paper of the like.
However, in the above-described prior art, a sufficient solid content rate cannot be achieved after the squeezing. Therefore, when the prior art are used to form a color image, it is impossible to form a latent image or color mixing occurs, and thus there is a disadvantage that two or more color toner images cannot be superposed on the latent image carrying member before the transfer step.
Further, since the toner layer developed on the latent image carrying member is electrically charged, there is observed such a phenomenon that the mirror image charge based on the charge of the toner layer itself is formed at the squeeze roller side, and toner is offset to the squeeze roller due to the electric force between the charge of the toner layer itself and the mirror image charge. This phenomenon is mainly caused by the fact that at the squeezing portion, an external electric field, that is, an offset-preventing potential difference is not provided between the latent image carrying member board and the squeeze roller conducting portion, or the value of the offset-preventing potential difference is improper even when the offset-preventing potential difference is provided. Particularly when the potential setting is carried out as described in Japanese Laid-open Patent Application No. Hei-10-73997, a large amount of toner adheres to the squeeze roller side, and a cleaning work for the toner is more cumbersome. Further, since the toner is withdrawn from the image portion to the squeeze roller, an image thus formed is short of image density. In addition, a squeeze offset phenomenon is liable to occur.
An object of the present invention is to provide a wet-type developing apparatus which can simultaneously perform sticking and cohesion of toner particles developed in an image portion area on a latent image carrying member, removal of surplus carrier liquid and removal of a liquid toner layer adhering to a non-image portion area, and also can achieve an image having sufficient image density with no occurrence of toner dispersion and flow-out of toner, and an image forming apparatus using the wet-type developing apparatus.
Further, another object is to provide an image forming apparatus which can form multi-color images on a latent image carrying member and collectively transfer these multi-color images, and other object is to provide a wet-type developing apparatus and an image forming apparatus in which no offset phenomenon to a squeeze roller occurs.
In order to attain the above objects, according to the present invention, a wet-type developing apparatus In which a latent image carrying member and a squeeze member (such as a squeeze roller and a squeeze belt) are moved in the opposite directions to each other without coming into contact with each other and a liquid toner layer on the latent image carrying member is squeezed between the latent image carrying member and the squeeze member, comprises means for forming a liquid toner layer on the latent image carrying member so that the liquid toner layer has a thickness smaller than the gap between the latent image carrying medium and the squeeze member; means for making the liquid toner layer invade into the gap together with the latent image carrying member; and means for forming an electric field between the squeeze member and the latent image carrying member, wherein the liquid toner layer invading into the gap between the squeeze member and the latent image carrying member is brought into contact with the squeeze member by the electric attractive force based on the electric field to form a desired meniscus.
In the wet-type developing apparatus thus constructed, the squeezing means for squeezing the surplus carrier liquid of the liquid toner layer adhering to the surface of the latent image carrying member in the developing area is moved in the opposite direction to that of the latent image carrying member. The thickness of the liquid toner layer adhering onto the latent image carrying member is smaller than the gap distance at the closest position between the squeezing means and the latent image carrying member, and it is set to such a value that the liquid toner layer is not brought into contact with the squeezing means when the electric field based on the potential difference between the latent image carrying member board and the conductive layer of the squeezing means is not applied. At this time, by applying the electric field between the squeezing means and the latent image carrying member, the liquid toner layer on the latent image carrying member is brought into contact with the surface of the squeezing means. The surplus carrier liquid is squeezed out by the shearing stress which occurs in the liquid toner layer due to the speed difference between the latent image carrying member and the squeezing means, and the solid content (component) rate of the toner layer on the latent image carrying member passing through the squeezing means can be increased, so that a clear image can be achieved by transferring this toner. Further, in the color image forming process, the toner transfer can be excellently performed without mixing colors in the next developing step for the second and subsequent color toner images.